OBJECTIVES: Although hyponatremia (HN) is the most common electrolyte abnormality in hospitalized patients, little is known regarding the influence of HN and its management on patient outcomes and healthcare resource usage. The HN Registry is a novel prospective effort to document the clinical and healthcare outcomes of HN and its management. Results for the first 25 HN patients enrolled are described here.   METHODS: After informed consent or waiver, data were extracted from medical charts of enrolled patients. HN was defined as a serum sodium ≤ 130 mmol/L. The pilot data were summarized appropriately by sample size and for categorical data by percentage. Subjects who had HN on admission were categorized as pre-existing HN patients and those who were admitted for another reason and developed HN while in the hospital were categorized as hospital-acquired HN patients.    RESULTS: Overall, only 20% of the enrolled patients received any pharmacologic management for HN and approximately 44% were discharged with persistent HN (21% of treated vs. 79% of untreated). Among the patients discharged with HN, 55% had a previous episode of HN. In addition, among the patients with previous HN, 46% were discharged with persistent HN. The length of stay for patients with pre-existing HN was 1.3 days longer compared to patients with hospital-acquired HN.  These findings will be further evaluated and reported as more data in this large registry study are accumulated. CONCLUSIONS: Among hospitalized patients, HN is frequently untreated, and nearly half of patients are discharged without normalization of serum sodium. HN commonly persists through several hospital admissions.